A voltage source inverter (VSI) is used to generate an AC output voltage with controllable magnitude and frequency. The VSI converts a DC voltage to the AC voltage in order to drive a three phase AC motor. Pulse width modulation (PWM) methods are used to control the operation of the VSI. A PWM control module provides high-frequency voltage pulses that control a duty cycle of one or more switches in the VSI.
Referring now to FIG. 1, an exemplary three-phase VSI 10 that drives an AC motor 12 is shown. The VSI 10 includes one or more DC voltage sources 14 and 16 and switches 18-1, 18-2, 20-1, 20-2, 22-1, and 22-2, referred to collectively as switches 24. The switches 24 are any suitable semiconductor switches as are known in the art, such as transistors including an antiparallel diode. Typically, each switch pair is operated in a complementary manner. For example, when the switch 18-1 is ON, the switch 18-2 is OFF. Conversely, when the switch 18-2 is OFF, the switch 18-1 is ON. The other switch pairs are operated in an analogous manner. The AC motor 12 is a three phase AC motor. In other words, the AC motor 12 operates according to current flow through inductor coils 26, 28, and 30. Current flows through the inductor coils 26, 28, and 30 according to the voltage sources 14 and 16 and ON and/or OFF statuses of the switches 24.
A PWM control module 32 generates one or more switching control signals 34 that control switching behavior of the switches 24. The PWM control module 32 implements a known PWM control method that operates the switches 24 to achieve desired performance characteristics of the AC motor 12. For example, PWM control methods may be continuous or discontinuous. Continuous PWM control methods, such as Sinusoidal or Space Vector modulation, cause each switch in a phase leg to cycle (i.e. turn ON and OFF) once per cycle of a carrier signal. Discontinuous PWM (DPWM) control methods cause one switch in a phase leg to remain ON or OFF continuously for a portion of a cycle of a voltage fundamental signal. A modulation signal (e.g. a switching control signal) determines a duration that the switch is ON or OFF. Typically, DPWM control methods result in lower inverter losses than continuous PWM control methods.
Three conventional DPWM methods are DPWM0, DPWM1, and DPWM2 as shown in FIGS. 2A, 2B, and 2C, respectively. Referring now to FIG. 2A, a DPWM0 waveform illustration 40 includes a voltage fundamental signal 42, a modulation signal 44, and a current signal 46 during a phase a of AC motor control. An amplitude of the modulation signal 44 is representative of a commanded duty cycle of one or more of the switches 24. In other words, the modulation signal 44 controls the duty cycle of the switches 24. Typically, the DPWM0 control method is loss optimized for lagging power factor loads. For example, the DPWM0 control method may be preferred for AC motors operating in a regeneration mode.
Referring now to FIG. 2B, a DPWM1 waveform illustration 50 includes a voltage fundamental signal 52, a modulation signal 54, and a current signal 56 during a phase a of AC motor control. The DPWM1 control method is loss optimized for unity power factor loads. For example, the DPWM1 control method may be preferred for motors operating at low speeds or with light loads.
Referring now to FIG. 2C, a DPWM2 waveform illustration 60 includes a voltage fundamental signal 62, a modulation signal 64, and a current signal 66 during a phase a of AC motor control. The DPWM2 control method is loss optimized for lagging power factor loads. For example, the DPWM2 control method may be preferred for motors in motoring action.